1. Technical Field of the Invention
The present invention pertains in general to the selection of a cell in a cellular telephone system and, more particularly, to a method and apparatus for selection of a standby state cell in a cellular telephone system having a layered cell architecture.
2. Description of Related Art
The following descriptions use the Global System for Mobile communication (GSM) protocol by way of example. It is understood, however, that the issues, concepts and solutions provided by the present invention also apply to other wireless communication systems and protocols.
In cellular telephone systems a geographic service area is divided into a plurality of individual regions referred to as cells, each of which is provided with cellular telephone service on a plurality of separate communication channels the frequencies of which are reused among different cells separated from one another such that there is a sufficiently low level of interference between them. When a mobile station operating within the cellular telephone system is in a standby state, for example immediately after powering up, it searches for a cell and its associated broadcast control channel to lock onto. To select a cell, the mobile station scans radio frequencies in the cellular telephone system and measures downlink signal strength levels for each of them. The mobile station tunes to the strongest carrier frequency and determines if it is a broadcast control channel. If so, the mobile station locks onto the broadcast control channel associated with the cell. If the carrier is determined not to be a broadcast control channel, the mobile station tunes to the second strongest carrier and repeats the process. In such a fashion, the mobile station eventually locks onto the cell with the strongest broadcast control channel downlink signal strength.
As the demand for cellular telephone service increases over time, the capacity of existing systems has been severely stressed to serve all the subscribers who desire cellular telephone service, particularly in major metropolitan areas. For example, in the area in and around a convention center, the number of mobile stations may be so great that the demands for service cannot be satisfied by the entire channel capacity of the base station serving the cell within which the convention center is located. In such situations, additional "layers" of cellular telephone service is provided by additional lower powered base stations located within an existing, so-called "umbrella," or "macro" cell, and referred to as "microcells". Such microcells may have a coverage or service area on the order of a few hundred meters in contrast to a few kilometers of coverage of the overlying umbrella cell. A plurality of such microcells may be located adjacent to one another and form a contiguous coverage area of substantial width all of which is within the overall coverage area of the umbrella cell. Alternatively, they may be separated from one another within the umbrella cell.
When a layered cell structure is used, there is provided an enhanced level of service capacity which can be configured for individual circumstances and which provides an assurance that users can receive service despite an extremely high demand within a very small geographic area. Moreover, additional layers of cellular telephone service may be added, for example, by a plurality of either contiguous or separated "picocells" positioned within the service area of the individual microcells, each of which are in turn within the overall umbrella cell. The base stations providing the cellular telephone service within the picocells are of even still lower power than the base stations serving the microcells and have a service area of, for example, a few hundred meters to provide coverage within a single building or a single floor within a large convention center.
In layered cell architectures, many more options are available to a mobile station when selecting a cell to lock onto. For example, the mobile station may receive cellular telephone service at any given moment from either a picocell base station, a microcell base station, or an umbrella cell base station. When conventional criteria for standby state cell selection in a single layered cellular telephone system architecture is applied to a layered architecture, the results are less than ideal. When standby state cell selection is performed in a single layer cellular telephone system architecture, the principle criterion used is the strength of the downlink signal received by the mobile station from the respective base station broadcast control channel. In a layered architecture the strongest broadcast control channel is most likely to be associated with the highest level layer or umbrella cell since the umbrella cell transmits with the greatest power to serve a relatively large geographic area. In a layered architecture, however, it is undesirable for a mobile station to lock onto the highest level layer cell since it is generally preferable to serve the mobile station with the lowest possible layer cell for capacity reasons.
In order to maximize capacity of the cellular telephone system, mobile stations are typically instructed by the cellular telephone system to place and receive telephone calls on the lowest level cell possible. Thus, if sufficient signal quality exists from a lower level cell, such as a microcell or picocell, it is preferred to serve the mobile station from that cell rather than the umbrella cell which has fewer total channels available for service as compared to lower level cells given the geographical size of the service area. Using the conventional single layer standby state cell selection, however, the mobile station locks onto a higher level cell than is desirable for carrying a telephone call. Thus when the mobile station places or receives the telephone call using the conventional criteria for standby state cell selection for a single layer cellular telephone architecture in a layered cellular telephone architecture, the cellular telephone system performs a handover from the higher level cell which the mobile station has selected to a lower level cell. The handover increases processor capacity requirements and slows down the call setup procedure.
Because of the foregoing problems associated with the use of the existing standby state cell selection method within layered cellular telephone system architectures, it is desirable to introduce a method and apparatus for standby state cell selection which maximizes the efficient utilization of channel availability within a layered cellular telephone system. The system of the present invention provides such a method and apparatus.